The invention relates to an apparatus and a method for separating particles of material of different electrical conductance. More specifically, the mixed particles are fed onto a conveyor and separated by a rotating magnetic system arranged on the conveyor. A collecting container collects the sorted particles that are separated from the remaining material on the conveyor.
In an apparatus of this type known from U.S. Pat. No. 3,448,857, a quantity of particles of different electrical conductance, which have to be sorted, is fed onto a conveyor belt from above. The conveyor belt turns over a belt drum and takes the particles to that drum at a speed of 1 to 1.5 m/sec. A magnetic system rotates in the belt drum at a speed of approximately 1500 r.p.m. A relative movement takes place between the conveyor belt and the drum with the magnetic system during operation, and the difference in speed makes the magnetic lines of force cut through the electrically conductive particles traveling on the belt. Currents are thereby induced of a strength dependent on the electrical conductance of the particles. A stronger current is generated in the particles with higher electrical conductance, and causes those particles to be thrown in a trajectory from the belt into their direction of movement. The particles with lower electrical conductance remain near the belt and drop off it almost vertically. The exact fraction, which has a certain required electrical conductance, may be filtered out by installing a collecting container at a suitable location. It should be noted that before the ""857 apparatus is used, ferromagnetic materials have already been picked out of the material to be sorted by methods which are sufficiently well known (strong magnets). The main function of the ""857 apparatus is in fact to separate so-called nonferrous metals (copper, aluminum, lead, zinc, brass etc) from residual materials (paper, plastic, glass etc) particularly in connection with waste recycling.
An apparatus for separating mixtures of materials with different electrical conductance including a similar rotating magnetic means is known from DE 34 16 504 AI. In this apparatus, the magnetic means rotates rapidly and produces a changing magnetic field through which the mixed particles are passed. The separating means is surrounded by a case, which rotates more slowly. The eddy currents arising have effects on the particles, giving the electrically conductive particles a greater trajectory than the non-conductive ones.
PCT Patent Publication No. WO 89/07981 shows a comparable construction. Here again materials made up of non-magnetic particles drop from above onto a rotating drum containing a magnetic system that also rotates. The drum and the magnetic system rotate in opposite directions so that non-metallic materials such as glass, plastic and stones drop down on one side of the drum and non-magnetic metals on the other side. However, constructions in accordance with DE 34 16 504 AI or WO 89/07981 only allow non-specific separation, and the number of incorrectly separated particles is relatively high. Magnetic particles, which have not been previously separated out, also pose a problem, and may cause damage on going between the drums and the cases rotating in the other direction.
As a further improvement to such apparatus EP 0 339 195 BI proposes that the magnetic system should be arranged eccentrically in the belt drum. This arrangement prevents magnetisable, electrically conductive particles from being held between the conveyor belt and the belt drum. These trapped particles become red hot as a result of the magnetic field and inflict corresponding damage to the belt drum and conveyor belt. Another eccentric arrangement is shown in Japanese Patent Publication JP57-119856 A.
It has already been proposed in German Patent Publication DE 4 323 932 CI that the speed of the drum of the magnetic system should be raised and the deflection thus made stronger in order to improve sorting quality. However, this necessitates correspondingly expensive improvement of the properties of the magnetic system.
The problem of the invention is to provide an apparatus of the above type and a corresponding method, which improve sorting quality even without increasing the speed, or if the speed is increased, also increase the quality of the sorting.
The problem is solved in the present invention by providing an apparatus wherein the rotary direction of the magnetic system is chosen so that the directions of movement of the surface of the magnetic system and of the conveyor are different.
The problem is solved in the present invention by a sorting method wherein the surface of the magnetic system and the particles to be sorted are moved in different directions.
With the various components in such relative positions the sorting quality can be decisively improved. In known metal separators, the conveyor belt is used only to bring the particles which have to be sorted to the actual sorting point, i.e. the magnetic system; the magnetic system then decides, from the size of the parabolic trajectory formed, whether the particle is to be regarded as electrically more conductive or less conductive and hence whether it shall drop into a certain collecting container or not. This may sometimes cause problems and incorrect assessments, for example when particles lie over or cover each other and thus interfere with each other through the flight parameters.
In the present invention, the highly electrically conductive particles move in a different direction from the less conductive particles (not just to a different degree in the same direction, as in prior art); the limit can be set very sensitively here through the strength of the magnetic field of the magnetic system and/or the speed of the conveyor belt. The conveyor belt causes a basic movement of all the particles in a certain direction, and this is counteracted by the magnetic field of the magnetic system. The magnetic field may be made strong enough to move the highly conductive particles against the action of the conveyor belt, in the opposite direction, without any problems. In one embodiment, the start of the parabolic trajectory begins directly above the magnetic system such that some of the affected particles will have no further contact with the belt if they are caught and deflected sensitively enough above it.
In another embodiment, a certain section of conveyor belt is thoroughly and deliberately taken into account. Here again the magnetic field is found to be strong enough to convey the particles over the end of the upper run into a collecting container installed there.
If particles, possibly of different specifications, are on top of each other, mixed up or possibly diffused into each other, they will be disentangled or also spun to and from on the conveyor belt by forces acting in different directions, and will consequently be detached from each other; the effect can be seen immediately in the case of superimposed particles.
Instead of simply traveling along subtlety different parabolic trajectories, the particles move in diametrically opposite directions and therefore cannot interfere with each other.
If two particles landing in adjacent positions move diametrically towards each other and meet, this still does not diminish the sorting quality. After the impact, they will reappear unchanged in the same position, but in a slightly different relative arrangement, and will then automatically move in the right direction at the second attempt. Conveying and sorting of a particle in an incorrect direction is thus prevented.
In contrast with constructions where a case rotates around the magnetic system instead of an additional conveyor, the sequence in the present invention is that the particles to be separated are given a finite dwell time in the magnetic region, during which they are still accessible to decisions and influences. If a second drum is used instead of the conveyor, any initial incorrect classifications will generally be maintained, e.g. because pairs of elements may be hooked together.
A conveyor other than a belt may be used, e.g. a channel conveyor on which the particles are moved forwards by vibration or simply by gravity. Here the effects are similar.
A feed means is preferably provided for supplying the material to be sorted to the conveyor. The feed means may itself be a belt or channel conveyor. Again, it is preferable for the area adjacent the feed point to be made of a nonconductive material such as a plastic. In this way, a certain influence is exerted on the particles of material shortly before the feed point, and their dwell time under the influence of the magnetic system, which is conducive to their precise movement and assessment, is further lengthened. The conductive particles are found to orient themselves even while they are dropping onto the feed point and to move purposefully in the desired direction even before they land there.
In a particularly preferred embodiment, the directions of movement obtained in this manner are substantially perpendicular to each other. This means that the axis of rotation of the magnetic system is substantially parallel to the conveying direction of the belt or channel conveyor or alternatively at a relatively small angle thereto; the magnetic system thus rotates across and below the conveyor, thereby moving its surface substantially perpendicular to the direction in which the particles in the channel conveyor are conveyed. As a result, the forces acting on the nonferrous metals lead to strong movement of those metals, driving them away from the channel conveyor or to one side of it, while the ordinary non-metallic particles are unaffected. This effect may be utilized to drive non-ferrous metals down over the edge of the belt or channel conveyor and catch them in a collecting container there. Combinations of these various ideas are also possible.
The idea behind all the above-mentioned rotating magnetic systems is merely to separate all the non-ferrous metals from other waste such as glass or plastic and thus allow justifiable recycling. The accuracy of the operation did not permit any other separation. The parabolic trajectories of heavy but highly conductive copper parts and those of light but relatively non-conductive aluminum parts, or even those of pieces of glass with rolling motion components are similar and merge into each other, thus posing a problem in the past.
Separation of different metal components in a material to be sorted is therefore avoided as far as possible or, if absolutely essential or required, is carried out manually or by very expensive processes such as flotation in suitably conditioned liquids with precisely set specific densities. However, apart from being costly, this produces polluted liquids, which are expensive to dispose of, and wetted non-ferrous metals (again with problem constituents which have to be cleaned off).
According to the invention however, even these non-ferrous metals can be separated from each other. This is particularly possible in an embodiment where the conveyor has a conveying direction substantially parallel with the axis of the rotating magnetic system, and the conveying takes place in a channel conveyor above that system. The channel conveyor is preferably arranged not centrally above the magnetic system, but slightly offset, even if it overlaps it.
The channel conveyor should be slightly inclined transversely to the conveying direction, with the lowest point at the side remote from the magnetic system. The side towards the magnetic system or towards the centerline in its surface is either open, or forms an accumulating edge.
This particular shape enables the relative differences e.g. between two nonferrous metals to be utilized. For example, copper has high conductance and relatively high specific gravity, whereas aluminum has relatively low conductance and also low specific gravity. So copper particles are relatively difficult to accelerate despite the strong influence of the magnetic field. In fact it is found in practice that, if the channel conveyor is appropriately inclined, and also laterally and vertically spaced from the central axis, all the aluminum particles can be sluiced out from the side of the conveyor across the magnetic system before the copper particles are also sluiced out.
This is aided by the fact that the particles of material to be sorted tend to accumulate at the lower edge of the channel conveyor; that is to say, small, easily accelerated particles move along further away from the magnetic field, while larger and thus heavier and less easily accelerated particles continue to extend into it.
This also applies to mixtures including other components. Not only do aluminum particles appear relatively frequently in standard material requiring sorting, but out of all the materials studied they have the most favorable combination of specific gravity (or more precisely density, i.e. ratio of mass to volume) and conductance; they are therefore sorted out of such non-ferrous metal material first, after which the parameters are changed until they are sufficient for sluicing out the next component in question, and so on. This may be done not only by passing the material through repeatedly but also by arranging a sequence of different channel conveyor sections, so that different non-ferrous metals are then sorted out in turn in different lengths of the rotating cylindrical magnetic system. A further opportunity is obtained if the cross-section of the channel conveyor across the conveying direction has a non-level base, particularly a base with the highest point in its central region. With cross-sections of this type which are not level in the conveying direction an additional effect is obtained which helps to disentangle particles and also tends to separate them according to the ratio of density to specific conductance. Initial slightly incorrect orientation of the particles caused by hooking together can easily be rectified however, merely by turning them slightly, as the particles are under the influence of the various forces for a certain time.
It is particularly preferable for the base to match the shape of the drum. If the drum rotates with the magnetic system, with its axis parallel with the conveying direction, the base, curved upwardly as a segment of a circle, may be arranged a relatively short distance above the drum. This means in particular that the magnetic field may be exploited very effectively, that is to say, it is used particularly effectively and may be made relatively smaller and also obtain the same effect.
Another preferred effect is obtained if a fluid is applied in a region above the magnetic system, as an addition to these or other embodiments. The various materials can thus be separated in a still more detailed way, particularly if the fluid is applied in doses, perhaps from an air nozzle, and especially when the constituent materials of the particles are already known from the charge supplied, so that the forces which will be generated by the magnetic system, the conveying action of the channel conveyors or belt and by the air nozzle or other fluid applying means (on the basis of the specific weight and shape of the particles) can therefore be specified. The effect may be improved by providing a rotating magnetic system both above and below the conveyor. The axes of the two systems are parallel and the rotary direction is such that the direction of movement in the surface regions of the two systems facing each other is the same. The magnetic field formed there is thus particularly stable and unambiguous for the particles, which pass through between the systems on the conveyor. A particular effect of this arrangement is that particles which already have a certain motion component of their own or are difficult to bring under control, e.g. because they rebound in an irregular manner, can also be sorted reliably.
One of the basic ideas of the invention is that the dwell time of a single particle of the material in the magnetic field should be lengthened as much as possible. In prior art the dwell time is an extremely brief moment during which the particles drop onto the conveyor belt from above, whereas in the present invention, the time is considerably lengthened so that the particles have far more opportunity to move into the correct sorting path in a structured manner under the influence of the controlling magnetic field.
Other features and advantages shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.